Princeton Chorus Breaks up on clean channel, chorus gets stuck...

[animal69]

Hi I have a 1990 Fender Princeton Chorus and the Clean channel is pretty honky and nasty, it almost sounds like a marshall EL34 amp with some really old speakers, but I'd rather have it sound clean like its supposed to. Also, the chorus often gets stuck in the on mode. The red chorus light stays lit whether or not chorus is activated. If chorus is activated, sometimes it modulates only a portion of the time and gets stuck at other times (likewise the little pulsating light that's supposed to represent the modulation and speaker panning will get stuck at a specific brightness). Does this sound familiar to anyone?

[oc disorder]

Well I would start at the power supply check those bigger white ceramic zener
dropping resistors to see if they have come unsoldered or gone open circuit.
On the attached diagram R 106 and R107. Then check the zener's themselves
should get similar readings on a multimeter comparing the two.
I suspect one of them is not suppling the +'ve or -'ve 16 volts the chorus and
preamp require.

[animal69]

Litte Problem -> Big Problem!

I checked them. They were within spec for resistance. RC106 passes 9.6V and RV 107 passes 9.9V.

Oh hell... .I tried measuring the current with my voltmeter expecting to see 100mA across and R106 across 67mA. You're supposed to measure current across bare wire only, right? If so I forgot that. Since I got zero I turned my meter up to the 10A reading and when I tried reading across the resistors there was a pik spark at one end of th resistor. Now theres no sound from the amp.. And the fuse is still intact! Fuck I shouldnt have done that ! The resistors still read at the correct resistance ratings. At first they read at infinity but I think they cooled down or something. Still, there's no sound from the amo.

So, it does power up, there's just no sound. The chorus light still does its, undulate-pause-undulate thing.

Now what do I do? This could be a great learning experience trying to fix the amp, assuming someone can help, or a chance to finally sell the amp as "nonworking" without the ambiguity.

[cjlectronics]

I had one in for repair a while back and it was doing the same thing. The one I had was a solid state amp, I assume yours is as well??

The problem was the potentiometers were bad. The Treble and Bass pots, when fiddled with, would cut the output in and out. I repaired it by replacing some of the pots and opening others and cleaning the element on those. The chorus LED stayed on, just like yours is doing, and cleaning the pots fixed that as well.

You may want to check the pots??

CJ

[oc disorder]

whoops..........
sounds like the power supply was ok before the "current" test.....
Now may be a good point to stop and take it to an experienced tech.

well assuming the schematic for the power supply is the correct one
and using a digital meter set on aprox 100v DC with the black probe on
ground - at TP3 should get aprox 27volts dc (TP= test point) and at
TP5 should get 16 volts dc.
Your inital measurement of 9.6volts dc across the dropping resistor
sounds about right depends on power to transformer and meter accuracy in theory 11 volts.
Also TP4 should be - (minus) 27volts DC and TP6 - 16volts DC.

Regarding measuring current normally a circuit must be broken and the
meter inserted in the break so the current flows through the meter
completing the circuit.
In this case if you want to know current and you know resistance and
voltage... use ohm's law.. current in amps = volts divided by resistance
not an ammeter.
By having the meter set to 10 amps its behaves like a short circuit between the probes and unfortunatly "measuring across the resistor" has meant its shorted it out .
As the fuse hasn't gone (the export model has low voltage fuses) and now there is no sound , with the power off and meter on diode test check CR 15,16 ,17,and CR 18 and the two Zeners CR19 and CR20.
Its also possible a piece of copper track on the other side of the circuit board has acted like a fuse and burnt out between the resistor and the diodes (CR 15-18).
Presume speakers are reconnected?
The reason the resistors first read infinty..would be because the power capacitors would still have charge discharging through the resistors and upsetting the meter set on resistance.Once the voltage had gone on the resistors the resistance reading returned to normal.

If you have power on with the chassis removed make sure you stay away from where the wall power comes in and the live fuse and terminals.
They are lethal.
Also check the speaker leads do not touch and short.
My inital post was intended to suggest measurements with the power off.
Once its going again I'de do what Cjlectronics suggested and clean or replace the pots.
Gtr tech suggestion below refers to IC 13 a TL072.
This doesnt explain the distortion in the clean channel...
Check the speaker wiring too the same colour to the respective
+'ve and -'ve terminals.
Like an earlier post re a Roland Jazz Chorus this would affect
the sound on both channels but doesn't explain the chorus modulation stopping.

[Gtr_tech]

A bad op amp in the oscillator will cause the chorus to "stick" and quit moduating. I had some Crate combo in a while back that did just that and an op amp was to blame. As far as the chorus sticking on (functionally) check the footswitch jack to make sure the contacts are clean and tight when no plug is in the jack. Also pull that board and check the solder on the jack pins.

[animal69]

whoops..........
sounds like the power supply was ok before the "current" test.....
Now may be a good point to stop and take it to an experienced tech.

well assuming the schematic for the power supply is the correct one
and using a digital meter set on aprox 100v DC with the black probe on
ground - at TP3 should get aprox 27volts dc (TP= test point) and at
TP5 should get 16 volts dc.
Your inital measurement of 9.6volts dc across the dropping resistor
sounds about right depends on power to transformer and meter accuracy in theory 11 volts.
Also TP4 should be - (minus) 27volts DC and TP6 - 16volts DC.

Regarding measuring current normally a circuit must be broken and the
meter inserted in the break so the current flows through the meter
completing the circuit.
In this case if you want to know current and you know resistance and
voltage... use ohm's law.. current in amps = volts divided by resistance
not an ammeter.
By having the meter set to 10 amps its behaves like a short circuit between the probes and unfortunatly "measuring across the resistor" has meant its shorted it out .
As the fuse hasn't gone (the export model has low voltage fuses) and now there is no sound , with the power off and meter on diode test check CR 15,16 ,17,and CR 18 and the two Zeners CR19 and CR20.
Its also possible a piece of copper track on the other side of the circuit board has acted like a fuse and burnt out between the resistor and the diodes (CR 15-18).
Presume speakers are reconnected?
The reason the resistors first read infinty..would be because the power capacitors would still have charge discharging through the resistors and upsetting the meter set on resistance.Once the voltage had gone on the resistors the resistance reading returned to normal.

If you have power on with the chassis removed make sure you stay away from where the wall power comes in and the live fuse and terminals.
They are lethal.
Also check the speaker leads do not touch and short.
My inital post was intended to suggest measurements with the power off.
Once its going again I'de do what Cjlectronics suggested and clean or replace the pots.
Gtr tech suggestion below refers to IC 13 a TL072.
This doesnt explain the distortion in the clean channel...
Check the speaker wiring too the same colour to the respective
+'ve and -'ve terminals.
Like an earlier post re a Roland Jazz Chorus this would affect
the sound on both channels but doesn't explain the chorus modulation stopping.

should i just check for resistance of 0 across one direction of the diode and infinity the other way or is there a special way to use my meter to test diodes?

[oc disorder]

Yep Should be low one way and high the other.
If it has a diode test function you should read .6 or .7 across the diode
due to the normal aprox half a volt dropped across a diode.
The small current running between the meter probes passes through the diode
easily in one direction and doesn't the other direction but can pass through
other components in the associated circuit.
Its common to see the capacitors charging up till the reading settles then reversing the leads (depending on the circuit) can see the cap discharging and recharging the other way..till it settles.
You wont get 0 ohms in one direction and infinity in the other .. but you should get similar readings comparing the diodes.